What clearances are required around transformers whey they're installed?

All dry-type transformers depend upon the circulation of air for cooling; therefore, it is important that the flow of air around a transformer not be impeded. UL 1561 requires that there be no less than 6 inches (152.4 mm) clearance between any side transformer with ventilated openings and any wall obstruction. Transformers should not be mounted in such a manner that one unit will contribute to the additional heating of another unit, beyond allowable temperature limits, for example, where two units are mounted on a wall one above the other.